The vapours pass between the inner and outer tubes.
A dissolved in B and B dissolved in A, since both of these solutions emit vapours of the same composition (this follows since the same vapour must be in equilibrium with both solutions, for if it were not so a cyclic system contradicting the second law of thermodynamics would be realizable).
A more general designation is "pyrogenic processes," which also includes such operations as leading vapours through red-hot tubes and condensing the products.
Sidney Young has suggested conducting the operation in a current of carbon dioxide which sweeps out the vapours as they are evolved, and also heating in a vapour bath, e.g.
In its simplest form the apparatus consists of a straight tube, made of glass, porcelain or iron according to the temperature required and the nature of the reacting substances, heated in an ordinary combustion furnace, the mixture entering at one end and the vapours being condensed at the other.